The invention relates to a power-assisted steering with an electric motor and a superimposing gear mechanism, which superimposes a correction angle on the steering angle that is specified by the steering handle. In such a power assisted steering, the gear ratio of the steering transmission is not a fixed quantity and may be set variably, depending on different parameters, such as the vehicle speed and the driving situation that is arising.
The DE-A1-19801826 discloses such power-assisted steering. A steering system is described there, which has an electric motor and a superimposing gear mechanism, which is controlled by the electric motor and superimposes a correction angle on the steering angle specified at the steering handling device of a vehicle. An auxiliary turning motion is superimposed on the turning motion brought about by the driver. Moreover, the rotor of the electric motor is connected with a mechanical part of the superimposing gear mechanism, such as a worm gear or a skew gear, over which the movement, generated by the electric motor and superimposed on the manual turning movement, is fed into the steering. The extent of the superimposing depends, for example, on the actual speed of the vehicle. With such power-assisted steering, it is possible to vary the transmission ratio very flexibly. For example, it is very helpful, if the gear ratio is smaller when the vehicle is moving quickly than when it is moving slowly or is being parked. Even a failure of the power assistance due to a malfunction of the electric motor or of its electronic control would not result in a dangerous situation for the driver, since the driver basically could steer the vehicle even without the help of power assistance.
However, defects or malfunctions, which cause an unintended movement of the motor and, with that, an unintended intervention in the power-assisted steering, cannot be excluded in the electric motor itself or in its electronic control.
In the DE-A-1000219, a power assisted steering with an improved superimposing gear mechanism is proposed, which has, among other things, a “self-locking worm gear” (refer to column 2, lines 18 to 30 as well as claim 13 there). This construction is used to reduce the backlash in the mutually meshing transmission parts. However, it is not mentioned there that such a self-locking gear mechanism can also help to reduce the problems mentioned above, namely, to counteract the risky effects resulting from faults or malfunctions occurring in the steering.
Practical experience has shown that, in the case of (an intentional or unintentional) failure of the servomotor, there is a high degree of uncertainty in steering a vehicle even if a self-locking worm gear is present. This means that a safety risk, which cannot be disregarded, remains particularly in the case of worm gears or other types of gear mechanisms, which do not have a self-locking feature.
It is therefore an object of the invention to provide power-assisted steering of the type given above with a safeguarding device, by means of which unintentional power-assisted steering is prevented independently of the of gear mechanism used.
Accordingly, the electric motor is provided with a mechanical lock, which deactivates the electric motor in addition to an electrical deactivation and which, at least in situations in which there is a malfunction, locks the mechanical part of the superimposing gear mechanism and secures it against movement, in that the gear mechanism part (such as the worm, the worm gear or the skew gear) is provided with elongated hole-shaped grooves, which are engaged by a pin of the mechanical lock.
Advantageous developments arise out of the dependent claims.
The invention is now described in greater detail by means of an example, reference being made to the following diagrammatic drawings.